J. Phys. Chem. A 2010, 114, 3199–3206
3199
Infrared Spectroscopy of Jet-Cooled Tautomeric Dimer of 7-Azaindole: A Model System for the Ground-State Double Proton Transfer Reaction† Haruki Ishikawa,* Hiroki Yabuguchi, Yuji Yamada,‡ Akimasa Fujihara, and Kiyokazu Fuke* Department of Chemistry, Graduate School of Science, Kobe UniVersity, Rokko-dai, Nada-ku, Kobe 657-8501, Japan ReceiVed: September 29, 2009; ReVised Manuscript ReceiVed: NoVember 5, 2009
To investigate the ground-state double proton transfer (GSDPT) reaction, we carried out a laser spectroscopic study on the tautomeric dimer of 7-azaindole in a supersonic jet. We have recorded an infrared (IR) spectrum of the tautomeric dimer in the S0 state. The NH band exhibits a broad and less-structured pattern. The band pattern is discussed on the basis of the hierarchical vibrational interaction mechanism. As a result, much higher density of state (DOS) at the NH stretch level is expected than that of the normal dimer. Such a high DOS should be related to the anharmonicity of the potential energy surface near the barrier of the GSDPT reaction. To get more information, an N-D deuteration effect is examined. In the present experiment, five deuterated dimers are identified by visible-visible or IR-visible population labeling spectroscopy. The IR band pattern of the NH-ND dimer is very different from that of the NH-NH dimer. Among several N-D deuteration effects, a change in a condition between the inter- or intramolecular vibrational energy flows due to the single N-D deuteration is considered to be important. 1. Introduction The double proton transfer (DPT) reaction is one of the simplest chemical reactions and plays an important role in many reactions in various fields. Taylor and coworkers reported an excited-state double proton transfer (ESDPT) reaction of 7-azaindole (7-AI) dimer for the first time in 1969.1 In 1984, Fuke and coworkers reported the first observation of the ESDPT reaction of 7-AI dimer in the gas-phase cluster.2 They have succeeded in gaining detailed information such as a vibrational mode dependence on the DPT yield. Owing to the ESDPT reaction, a large number of spectroscopic studies on the 7-AI dimer in both the gas and the condensed phases have also been carried out until now.1-16 In addition to the experimental studies, many theoretical studies on the ESDPT reaction of the 7-AI dimer have been reported so far.17-20 The reaction scheme of the DPT reactions of this system is schematically described in Figure 1. Hereafter, we refer to two dimer configurations as the normal and the tautomeric dimers, as indicated in Figure 1, respectively. Although the ground-state DPT (GSDPT) process is also important in many chemical reactions, a GSDPT, that is, a reverse DPT reaction in the S0 state, has not been investigated as much as the ESDPT reaction. Until now, a few studies on the reverse DPT reaction of the 7-AI dimer system have been reported.5,8 Tokumura and coworkers carried out several spectroscopic measurements and reported the lifetime of the tautomeric dimer in 3-methylpentane solution to be 19 µs at room temperature. In addition, they obtained the activation energy of the reverse DPT reaction to be 1.4 kcal mol-1.5a Such a small activation energy suggests that an infrared (IR) excitation can initiate the GSDPT from the vibrational ground level of the tautomeric dimer. Therefore, the tautomeric dimer should be a †
Part of the “Benoît Soep Festschrift”. * Corresponding authors. E-mail:
[email protected] (H.I.); fuke@ kobe-u.ac.jp (K.F.). ‡ Present address: Department of Chemistry, Faculty of Science, Fukuoka University.
Figure 1. Reaction scheme of the double proton transfer reactions of the 7-AI dimer system. Molecular structure of the normal and the tautomeric dimers are also shown.
very good precursor or a model system to investigate the GSDPT reaction. Concerning the 7-AI tautomeric dimer, Fuke and Kaya have already succeeded in detecting the jet-cooled 7-AI tautomeric dimer and reported fluorescence excitation (FE) and dispersed fluorescence (DF) spectra.2b Owing to this spectroscopic information, we have carried out advanced laser spectroscopic measurements on this system. In the present study, we have measured IR spectra of jet-cooled 7-AI tautomeric dimer. The NH stretch motion is associated with the reaction coordinates of the GSDPT reaction. Therefore, it is expected that an influence of the GSDPT reaction should appear in the IR spectra in the NH stretch region. In general, band patterns of the vibrational spectra involve information about the vibrational interaction mechanism or vibrational dynamics such as the intramolecular vibrational energy redistribution (IVR) process.21,22 The former corresponds to an interpretation in a frequency-domain picture, whereas the latter is in a time-domain picture, respectively. In the case of the normal dimer, the vibrational interaction of the NH stretch mode has been interpreted in both time- and frequency-domain pictures.23-27 Sakai and coworkers carried out time-resolved IR spectroscopy of the normal dimer in the gas phase.24 Their timedependent spectra clearly revealed the energy flow from the initially excited NH stretch level to the other levels behind.
10.1021/jp909337w 2010 American Chemical Society Published on Web 12/03/2009
3200
J. Phys. Chem. A, Vol. 114, No. 9, 2010
Recently, a steady-state IR band pattern of the normal dimer is theoretically investigated by Dreyer.27 He has examined the vibrational anharmonic interactions of 7-AI normal dimer based on quantum chemical calculations. He has also demonstrated that the complicated band pattern can be reproduced by considering the anharmonic resonances of the NH stretch level with the levels having a character of the NH bend mode. Because vibrational frequencies of the modes of the tautomeric dimer should be different from those of the normal dimer, the different vibrational interaction scheme is expected. In the present article, we report an IR spectroscopic study on the 7-AI tautomeric dimer. A pattern of the NH stretch band is much different from that of the normal dimer. We discuss this difference based on a hierarchical vibrational interaction mechanism and also a relation with the GSDPT reaction. To obtain more information about the NH stretch dynamics, we investigate several deuterated dimers. In the interpretation of the deuteration effect, a localization of the vibrational motion is considered to play an important role in the vibrational dynamics. 2. Experimental Details In the present study, a conventional supersonic jet apparatus and a nanosecond pulsed laser system were used. The 7-AI tautomeric dimer was produced according to the method reported in ref 2b. The sample of 7-AI (Aldrich) was heated to 360 K to gain sufficient vapor pressure. The 7-AI vapor seeded in He buffer gas is supersonically expanded into a vacuum chamber through a pulsed nozzle with an orifice 0.8 mm in diameter. Just at an exit of the nozzle, UV (266 nm) laser light was irradiated to the 7-AI normal dimers formed in the jet. The UV light excites the normal dimer to its S1 state and promotes the generation of the tautomeric dimer by the ESDPT reaction. The tautomeric dimer produced in the S1 state relaxes to vibrationally excited levels of the S0 state by emitting a visible photon. The dimers are cooled to the vibrational ground level because of multiple collisions with the buffer gas at the nozzle exit. Therefore, the vibrationally cooled 7-AI tautomeric dimer is produced in the jet. Then, the tautomeric dimer is detected by the FE spectroscopy at 10 mm downstream from the point of the UV irradiation. This spatial separation corresponds to a delay time between the UV and the probe laser pulses of about 5 µs. Here the forth harmonic output of a Nd:YAG laser (New Wave Research, Polaris II) is used for the UV excitation of the normal dimer. The output is maintained at ∼1 mJ/pulse to avoid the photodecomposition of 7-AI in the jet. The visible (VIS) output of the tunable dye laser (Lumonics, HD-500) pumped by another Nd:YAG laser (Continuum, Surelite II) is used for the FE measurement. Delay times among the pulsed nozzle, UV, and VIS laser pulses are controlled by a digital delay generator (SRS, DG535). The brief description of the VIS-VIS28,29 or IR-VIS30 population labeling spectroscopy used in the present study is as follows. The population labeling spectroscopy is the same method as hole-burning spectroscopy, in principle. The wavenumber of the probe laser is fixed to the S1-S0 band origin, and the fluorescence intensity is measured. It reflects the population in the vibrational ground level. The VIS or IR pump laser pulse is irradiated to the tautomeric dimer in advance to the probe pulse, and its wavenumber is scanned. When the absorption of the pump laser pulse occurs, the population in the vibrational ground level decreases; then, the fluorescence intensity also decreases. Therefore, we can detect the VIS or IR absorption as the depletion of the probe fluorescence
Ishikawa et al.
Figure 2. Fluorescence excitation spectrum of the 7-AI tautomeric dimer recorded in the present study. The 0-0 band appears at 23 068.1 cm-1. Progressions of the intermolecular vibrations are indicated.
intensity. In the IR population labeling spectroscopy, the IR laser pulse was generated by the commercial opto-parametric oscillator/amplifier system (LaserVision). The IR laser pulse was irradiated to the 7-AI tautomeric dimer 20 ns prior to the VIS probe pulse. In the case of the VIS-VIS population labeling spectroscopy, output of another Nd:YAG (Continuum, NY-61) pumped dye laser (Spectra Physics, PDL-3) is used for the pump light. The delay time between the pump and the probe pulses was set to 150 ns to avoid the strong fluorescence signal due to the pump excitation. Wavenumber of the visible laser was calibrated by means of opto-galvanic measurements of neon atom. The wavenumber of the signal output (720-880 nm) of the IR-OPO/OPA system was also calibrated by the same method. The calibrated IR wavenumber is confirmed by the absorption spectrum of water vapor. In the present study, we have also carried out the same spectroscopic measurements on some deuterated species of the 7-AI tautomeric dimers. To generate the deuterated species, we added a few drops of D2O to the 7-AI sample. Deuterated 7-AI is produced by a rapid H-D exchange reaction. Using this partially deuterated 7-AI as a precursor, we have generated several deuterated dimers in the method described above. To obtain a theoretical support, we have carried out quantum chemical calculations on the 7-AI tautomeric dimer. We have performed structural optimizations and vibrational calculations at the B3LYP/6-31++G(d,p) or MP2/6-31++G(d,p) level using Gaussian 03.31 The vibrational calculations are carried out mainly at the former level. Vibrational frequencies of the deuterated species are also calculated. Harmonic vibrational frequencies obtained are scaled by a factor of 0.959. This value is estimated by the comparison between the observed and calculated values of the NH and the CH stretch modes of 7-AI monomer at the same level. We have also examined vibrational anharmonicities using an anharmonic option equipped in Gaussian 03. The anharmonicities are calculated on the basis of the third and fourth derivatives of the potential energy against the normal coordinates at the optimized structure. This anharmonic calculation has been carried out at the B3LYP/6-31+G(d,p) level to reduce computational loads. 3. Results 3.1. Fluorescence Excitation and Infrared Spectra of the 7-AI Tautomeric Dimer. Figure 2 shows a typical FE spectrum of the jet-cooled 7-AI tautomeric dimer obtained in the present study. This spectrum is essentially the same as that reported by Fuke and Kaya.2b The S1-S0 band origin appears at 23 068.1 cm-1. In the spectrum, three sets of progressions clearly appear. The main low-frequency (107 cm-1) vibration has been assigned as an intermolecular stretch mode, whereas the other two
IR Spectroscopy of 7-Azaindole Tautomeric Dimer
Figure 3. Infrared spectra of (a) the tautomeric dimer and, for comparison, (b) the normal dimer.
vibrations (73 and 80 cm-1) have been assigned as intermolecular bend modes. Using the S1-S0 origin band as a probe transition, an IR absorption spectrum of the 7-AI tautomeric dimer is recorded for the first time. An IR spectrum observed in the region from 2400 to 3200 cm-1 is shown in Figure 3a. Although we have measured IR spectra up to 4000 cm-1, there is no band in the region higher than 3200 cm-1. The IR spectrum of the normal dimer is shown in Figure 3b for comparison. The IR spectrum of the normal dimer is essentially the same as that reported by Yokoyama and coworkers.23 It is clearly seen that one strong and broadband appears centered at 2680 cm-1 in the spectrum of the tautomeric dimer. In addition to this strong band, several sharp bands appear at ∼3100 cm-1. The latter bands are assigned as antisymmetric CH stretch vibrations. These values are common with the other aromatic molecules. Our theoretical calculation has predicted that only the NH stretch vibration has a frequency in the region from 2400 to 3000 cm-1. Therefore, we have assigned the former band as an antisymmetric NH stretch band. The vibrational frequencies obtained by the theoretical calculation are listed in Table 1. For the normal dimer, the low-frequency shift of the NH stretch due to the dimer formation is ∼500 cm-1, measured from that of the 7-AI monomer (3521 cm-1).23 In the case of the tautomeric form, the NH stretch frequency of 7-AI tautomeric monomer has not yet been observed. Our theoretical calculation has predicted it to be 3444 cm-1 (B3LYP) or 3408 cm-1 (MP2). Using these values, the low-frequency shift due to the dimer formation is estimated to be ∼800 cm-1. This large shift indicates that the hydrogen-bond strength is quite large in the case of the tautomeric dimer. It is noted that there is a substantial difference in the band pattern between the tautomeric and the normal dimers. Because only a few peaks are involved in the NH stretch band of the tautomeric dimer, we have decomposed it into three Gaussiantype peaks, as shown in Figure 4. Several minor peaks at 2680 cm-1 do not exhibit reliable reproducibility. Therefore, we have ignored such small peaks in the present analysis. The observed band is well reproduced with one broad and intense peak and two rather sharp bands. The peak positions and widths are summarized in Table 2. This difference will be discussed on the basis of the vibrational interaction below. 3.2. Fluorescence Excitation and Infrared Spectra of the Deuterated Dimers. To obtain detailed information about the NH stretch mode of the 7-AI tautomeric dimer, we have carried out FE and IR spectroscopies of its deuterated dimers. As mentioned in the Experimental Details, we prepared the deuterated species by the H-D exchange reaction between 7-AI and
J. Phys. Chem. A, Vol. 114, No. 9, 2010 3201 D2O. When D2O is added to the sample, FE band patterns change with time on the order of several hours. One of the FE spectra measured in the present study is shown in Figure 5. The FE spectrum without adding D2O to the sample is shown in the top trace for reference. It is clearly seen that several sets of the characteristic band pattern of the intermolecular vibrations of 7-AI tautomeric dimer newly appear in Figure 5a. For a clear identification of the band origins of the deuterated species, an expanded view of the 0-0 band region indicated by a horizontal arrow is displayed in Figure 5b. The band of the undeuterated species is labeled as X in the Figure. Two bands appear in the lower-frequency side of X, whereas three bands appear in the higher-frequency side. Bands of the deuterated dimers are labeled as A to E from the lower-frequency side in order. Transition energies of the 0-0 band of the deuterated dimers are summarized in Table 3 together with the isotopic assignments described below. We have carried out VIS-VIS population labeling spectroscopy to identify how many deuterated dimers appear in the spectrum. The results are shown in Figure 6. Bands A and B and C and D are partially overlapped in the FE spectrum. As shown in Figure 7, these dimers are clearly separated in the VIS-VIS population labeling spectra. On the basis of the results of the population labeling spectroscopy, it is confirmed that all bands appeared in the FE spectrum belong to the five deuterated species (A-E) and the undeuterated one (X). Hereafter, labels A-E and X will be used to identify both the deuterated dimers themselves and their origin bands. In general, it is well known that the rapid H-D exchange efficiently occurs at the N-H group. In this case, there should be three isomers; NH-NH, NH-ND, and ND-ND dimers. However, the number of the isomers observed in the present study is greater than three. This means that other hydrogen atoms are also deuterated. A detailed assignment has been carried out on the basis of the results of the IR spectroscopy as follows. We have measured IR spectra of these deuterated dimers by fixing the probe laser wavenumber at the electronic origin band of each dimer. Figure 8 summarizes the IR spectra of the deuterated dimers. For comparison, the IR spectrum of the undeuterated dimer (X) is also displayed in the top trace. It is clearly seen that band patterns in the NH stretch region are classified into three groups: (X, E), (A, C), and (B, D). As mentioned above, the number of possible deuteration patterns at the NH hydrogen is three: NH-NH, NH-ND, and ND-ND. Because dimer X is the undeuterated species, the dimer E is assigned as an NH-NH dimer. Dimers B and D do not exhibit any band in the NH stretch region. This means that these dimers have no NH group. That is, these dimers are ND-ND dimers. The rest of the groups, (A, C), also exhibit relatively strong bands in the NH stretch region. However, the band pattern of this group is different from that of the NH-NH dimer. Therefore, we have assigned the dimer A and C as NH-ND dimers. A difference between dimers in each group can be seen in the band pattern of CH stretch modes emphasized in the insets in Figure 8. Several bands appear in these spectra. However, the band patterns are different from each other. This means that the deuteration at CH hydrogen also takes place. It is known that a CD deuteration of the three-position of 7-AI is more feasible compared with the others.7b In general, such a deuteration efficiently occurs under the basic solvent. In the present experiment, no base was added to the sample. It is considered that a contact with metal (stainless steel) in the sample reservoir should provide a similar effect. Then, the C-D deuteration has occurred efficiently in our experiment. In the CH stretch region of the IR spectra, a band at 3123 cm-1 appears in the spectra of
3202
J. Phys. Chem. A, Vol. 114, No. 9, 2010
Ishikawa et al.
TABLE 1: Vibrational Frequencies of the CH stretch (νCH), NH(D) Stretch (νNH(D)), and In-Plane NH(D) Bend (δNH(D)) Modes Obtained by the Theoretical Calculation at B3LYP/6-31++G(d,p) Levela NH-NH dimer
ND-ND dimer
NH-ND dimer
assignmentb
frequency (int.)
assignmentb
frequency (int.)
assignment
frequency (int.)
νCH (ag) νCH (bu) νCH (ag) νCH (bu) νCH (ag) νCH (bu) νCH (ag) νCH (bu) νCH (ag) νCH (bu) νNH (bu) νNH (ag) δNH (ag) δNH (bu)
3125 3125 3102 3102 3092 3092 3086 3086 3057 3057 2585 2490 1650 1638
νCH (ag) νCH (bu) νCH (ag) νCH (bu) νCH (ag) νCH (bu) νCH (ag) νCH (bu) νCH (ag) νCH (bu) νND (bu) νND (ag) δND (bu)c δND (ag)c δND (ag)c δND (bu)c
3125 (0.0) 3125 (17.2) 3102 (0.0) 3102 (7.5) 3092 (0.0) 3092 (15.1) 3086 (0.0) 3086 (11.2) 3057 (0.0) 3057 (21.6) 1935 (3316.3) 1878 (0.0) 1120 (39.6) 1105 (0.0) 987 (0.0) 978 (64.3)
νCH νCH νCH νCH νCH νCH νCH νCH νCH νCH νNH νND δNH δNDc δNDc
3125 (0.2) 3125 (16.0) 3102 (1.5) 3102 (5.4) 3092 (2.3) 3092 (13.6) 3086 (4.8) 3086 (6.7) 3057 (0.0) 3057 (20.4) 2540 (3616.4) 1904 (1380.2) 1643 (113.2) 1109 (17.9) 983 (33.7)
(0.0) (15.1) (0.0) (6.4) (0.0) (17.0) (0.0) (11.9) (0.0) (19.0) (6689.7) (0.0) (0.0) (185.5)
a Vibrational frequencies are scaled by 0.959. The vibrational frequencies are in units of inverse centimeters. Values in parentheses are intensities of each vibrational transition in units of kilometers per mole. b In the case of the NH-NH and the ND-ND dimers, the symmetry of each mode is denoted in parentheses. c In-plane ND bend mode is mixed with several skeletal modes. Therefore, the modes having relatively large contribution of the ND bend are listed in the Table.
Figure 4. Spectral decomposition of the NH stretch band of the 7-AI tautomeric dimer. Peak positions and band widths are listed in Table 2.
TABLE 2: Peak Positions and Widths of Components of the NH Stretch Band of the 7-AI Tautomeric Dimera
a
component
peak position/cm-1
fwhm/cm-1
1 2 3
2534 2578 2676
34 34 245
Observed band is decomposed by three Gaussian-type profiles.
dimers X, A, and B but not in those of dimers C, D, and E. Because the dimer X does not have any deuterium atom, the band at 3123 cm-1 can be assigned as the CH stretch mode corresponding to the three-position of 7-AI. Our theoretical calculation also supports this assignment.32 Therefore, it is assigned that the dimers A and B do not have a CD group and that the dimers C, D, and E contain a CD group. We have carefully examined a long-time temporal change in the FE spectra during the deuteration, as shown in Figure S1 in the Supporting Information. The band E appears in advance to the ND-ND dimers, B and D. This indicates that the dimer E contains one CD group but not two. Therefore, it is tentatively assigned that the C, D, and E dimers contain one CD group. It cannot be determined which moiety in the dimer has a CD group in this stage.
Figure 5. (a) FE spectrum of the 7-AI tautomeric dimers when adding a few drops of D2O in the sample. A spectrum of undeuterated dimer is displayed in the top trace. (b) Expanded spectrum in the region denoted by an arrow. Five deuterated dimers appear in the spectrum where some bands are partially overlapped with each other.
4. Discussion 4.1. Vibrational Interaction of 7-AI Tautomeric Dimer. As shown in Figure 3, the vibrational band pattern of NH stretch mode of 7-AI tautomeric dimer is very different from that of the normal dimer. In general, vibrational band patterns appearing in spectra are determined by the anharmonic interaction among vibrational levels or IVR processes. Such band patterns are frequently interpreted by the following stepwise or hierarchical mechanism.21,22 In the zero-order picture, it is assumed that only one level has transition intensity. This level is frequently referred to as a bright level. The anharmonic resonance is caused by third, fourth, and/or higher coefficients in an expansion of the
IR Spectroscopy of 7-Azaindole Tautomeric Dimer
J. Phys. Chem. A, Vol. 114, No. 9, 2010 3203
TABLE 3: Wavenumbers of the 0-0 Band (ν0) of Deuterated Species of the 7-AI Tautomeric Dimers Observed in the Present Studya species
ν0/cm-1
X A B C D E
23 068.1 23 049.2 23 049.9 23 069.6 23 070.3 23 088.3
assignment NH-NH, NH-ND, NH-ND, ND-ND, ND-ND, NH-NH,
CH CH CD CH CD CD
a Isotope assignments of the deuterated positions are denoted as follows: N-D deuteration is classified as NH-NH, NH-ND, and ND-ND. Concerning the C-D deuteration, it is denoted whether the species involves the CD group or not because it is not determined which monomer unit involves the CD group.
Figure 8. Infrared spectra of deuterated species of the 7-AI tautomeric dimer. Isotopic species is denoted in each part. Insets at the right side of the Figure are emphasized spectra of the CH stretch region.
Figure 6. VIS-VIS population labeling spectra of the deuterated species of the 7-AI tautomeric dimer. The band used as the probe transition is denoted in each spectrum.
Figure 9. Schematic representation of the vibrational interactions of (a) the normal and (b) the tautomeric dimers.
Figure 7. VIS-VIS population labeling spectra of the deuterated species of the 7-AI tautomeric dimer in the region of the 0-0 band. Partially overlapped bands in FE spectrum (bottom trace) are well separated, as shown (top and middle traces).
potential energy surface function along the normal coordinates. The bright level interacts with nearby levels through anharmonic resonance interactions. Because of a mixing of the wave function of the bright level and interacting levels, the transition intensity is distributed among these levels. Then, the original band is split into several bands. This is the first step of the IVR. The levels that strongly interact with the bright level are called doorway levels. The second step is an interaction of the resulting
levels of the anharmonic resonances with bath levels. The interaction with the bath levels is reflected as a width of the each band of resulting level. In the statistical limit, widths of the resulting bands of the first step interaction become broad, and finally, the bands integrate into one broadband. In this limit, the density of state (DOS) is more important than the interaction strength.21 On the basis of the above discussion, the interaction scheme of the normal dimer can be expressed, as shown in Figure 9a. In the case of the normal dimer, the NH stretch level interacts with many levels through the anharmonic resonances, and the splitting into many bands occurs. However, the interactions in the second step are not so efficient. As a result, the band pattern becomes an overlap of many relatively sharp bands. In the case of the tautomeric dimer, the interactions in the second step are
3204
J. Phys. Chem. A, Vol. 114, No. 9, 2010
dominant processes in determining the band patterns. The broad and less-structured pattern indicates that the interaction with the bath levels is close to the statistical limit. Two relatively sharp bands correspond to the resulting levels of the first step, which do not interact with the bath levels efficiently, and they are minor processes in the vibrational dynamics of the NH excitation. The interpretation of the vibrational interaction is schematically expressed in Figure 9b. In the case of the tautomeric dimer, the second step is the dominant process in IVR, whereas the first step is dominant in the case of the normal dimer. In Figure 9b, the bar denoting the doorway level is drawn rather thick to stress that several doorway levels may be involved. Even if there are several doorway levels within the width of the NH band, the splitting of the band due to the anharmonic interaction among the zero-order NH and the doorway levels should be smeared out by the second-step interaction with the bath levels. In the case of the normal dimer, the number of levels involved in the first step is much larger than that in the case of the tautomeric dimer. This difference can be explained as follows. As Dreyer pointed out in ref 27, the combination levels involving the NH bend excitation can interact with the NH stretch level through the anharmonic resonances. In the case of the normal dimer, because the vibrational frequency of the NH bend is very close to those of the C-C stretch vibrations, mixings of the NH bend and the C-C stretch modes occur, and the character of the NH bend mode is distributed among the C-C stretch modes. Therefore, the NH stretch level can interact with many vibrational levels having the character of the NH bend motion. In addition, the NH stretch vibration frequency is nearly the twice that of the NH bend modes. Therefore, small energy gaps also enhance the anharmonic resonances. A situation of the vibrational frequency is somewhat different in the case of the tautomeric dimer. The in-plane NH bend frequency of the tautomeric dimer is calculated to be 1638 (bu) and 1650 (ag) cm-1, and they are not mixed with the C-C stretch modes. Moreover, energy gaps between the NH stretch level (∼2600 cm-1) and the NH bend overtones (∼3200 cm-1) are quite large so that the anharmonic resonance interaction should be weak in such an energetic condition. Because of these unfavorable conditions, the number of levels interacting with the zero-order NH stretch level should be small. The observed IR spectrum indicates that the interaction with the bath levels is important for the tautomeric dimer. It can be related to a high DOS. The DOS is estimated using the method described in ref 33. In this calculation, harmonic vibrational frequencies are used. The result is shown in Figure 10. The DOS at the NH stretch level of the normal dimer at 3050 cm-1 is ∼16 times larger than that of the tautomeric dimer at 2650 cm-1. However, our interpretation of the band pattern expects an opposite situation. Because our DOS calculation does not include the anharmonicity, the discrepancy should come from the anharmonicity of the potential surface. Our interpretation claims that the potential energy surface of the tautomeric dimer should be more anharmonic than that of the normal dimer because the local minimum corresponding to the tautomeric dimer is located higher in energy than the normal dimer and is close to the barrier of the GSDPT reaction. Therefore, it is indicated that the NH stretch level is located very close to the potential energy barrier of the GSDPT reaction in energy. The potential barrier height for the GSDPT reaction is estimated to be 605 or 901 cm-1 measured from the bottom of the tautomeric well by MP2/6-31++G(d,p) or B3LYP/6-31++G(d,p) calculation, respectively. It is found that
Ishikawa et al.
Figure 10. Calculations of the density of state (DOS) of the normal and the tautomeric dimers. The upper line corresponds to the normal dimer, whereas the lower line corresponds to the tautomeric dimer. The harmonic DOSs are calculated using a method described in ref 33.
the GSDPT barrier has a C2h structure. The level of our calculation is not high enough for an accurate estimation of the potential barrier, and there is the possibility of underestimation. However, these values support our interpretation. This result is not direct evidence of the GSDPT reaction; however, it is considered to be one of spectroscopic signatures for the GSDPT reaction of the 7-AI tautomeric dimer. Here we would like to compare the vibrational dynamics between the NH and the CH stretch modes. As one can see, the width of the CH stretch bands is much narrower than that of the NH stretch band. This means that the vibrational dynamics of the NH level is not in a pure or complete statistical limit where there is no mode dependence. It should proceed in a stepwise or hierarchical manner, as shown in Figure 9. The fact that only the NH stretch level exhibits fast IVR process indicates that this IVR dynamics should be related to the intermolecular process, such as the DPT reaction. 4.2. Deuteration Effect on the Vibrational Interaction. In this section, the deuteration effect on the vibrational interaction is discussed. There are three groups of dimers observed in the present study: NH-NH, NH-ND, and ND-ND dimers, as shown in Figure 8. The IR spectra in the NH stretch region indicate that an effect of the C-D deuteration on the vibrational interactions of the NH stretch level seems negligible. Because an IR spectrum in the ND stretch region cannot be observed in our experiment, differences between the NH-NH and the NHND dimers are discussed. As clearly seen in Figure 8, two peaks appeared in the IR spectrum of the NH-ND dimer. A definitive assignment is difficult in this stage. Tentatively, the stronger peak at 2547 cm-1 is assigned to the NH stretch band, whereas the second peak at 2680 cm-1 is assigned to a combination band of the NH stretch and the intermolecular stretch. The energy difference between these two bands agrees with the vibrational frequency of the intermolecular stretch reported as 127 cm-1 by the DF spectroscopy.2b The width of the NH stretch band of the NHND dimer (69 cm-1) is narrower than that of the NH-NH dimer. According to our theoretical calculation of the NH-ND dimer, the ND stretching frequency is obtained to be 1904 cm-1, whereas that of the in-plane NH bend is ∼1100 cm-1.34 These frequency changes affect the vibrational interaction in two different ways. The first one is an effect on DOS. The large decrease in the vibrational frequencies makes the DOS larger. It may enhance the statistical interaction and make the bandwidth broader. A large change in the vibrational frequency due to the N-D deuteration makes the energy gap between the NH stretch
IR Spectroscopy of 7-Azaindole Tautomeric Dimer level and the doorway levels larger. To compensate the energy gap, the order of the anharmonic resonance should increase. In this case, the number of effective doorway levels becomes smaller, and the band splitting may disappear. This is the second effect of the N-D deuteration. Therefore, the net effect on the vibrational interaction should be determined by a balance of these opposite effects. One should consider another factor of the single deuteration. In the case of the 7-AI tautomeric dimer, the single deuteration of NH hydrogen lowers the molecular symmetry from C2h to Cs. This change removes symmetry restrictions on the vibrational interactions. Therefore, the number of symmetry-allowed candidates of the doorway level may increase. However, in the case of homodimers, where the two monomer components are equivalent, we have to consider vibrational interactions, or in other words, vibrational energy flows within each unit or across these two units. In the case of the NH-NH dimer, the skeletal vibrations are represented as even or odd linear combinations of the monomer vibrations. This means that all vibrations are delocalized over the entire dimer. On the other hand, vibrations involving the NH or ND stretch and bends are mainly localized in each monomer unit of the NH-ND dimer. Our preliminary anharmonic vibrational calculations provide that the anharmonic coefficient of the Fermi-type resonance for the NH stretch-NH bend-NH bend modes is -224 cm-1, whereas coefficients for the NH stretch-ND bend-ND bend modes are ∼3 cm-1. This result is quite reasonable because it means that the energy flow within the monomer unit is much faster than that across the monomer units. That is, the single N-D deuteration should decrease the number of the levels that are candidates of the doorway levels. Because there are several factors that can be affected by the N-D deuteration, it is difficult to determine which one is the most important in the present case. However, the localization of the vibrational motions due to the single N-D deuteration should largely change the condition between the inter- or intramolecular vibrational energy flows. It is a very interesting problem in the vibrational energy relaxation and also in discussing the DPT reaction. 5. Conclusions To investigate the GSDPT reaction, we have carried out VIS and IR spectroscopic studies on the tautomeric dimer of 7-AI. We have succeeded in recording IR spectra of the tautomeric dimer in the S0 state for the first time. The NH band exhibits a broad and less-structured profile. The band profile is discussed on the basis of the hierarchical vibrational interaction mechanism. As a result, the high DOS at the NH stretch level is expected. It should be related to the anharmonicity of the potential energy surface near the potential barrier of the GSDPT reaction. To get more information, the deuteration effect has been examined. Five deuterated dimers are identified in our experiment. The IR band pattern of the NH-ND dimer is different from that of the NH-NH dimer. Because several effects of the N-D deuteration are expected, the major contribution cannot be determined in this stage. However, the change in a condition between the inter- or intramolecular vibrational energy flows due to the single N-D deuteration should be important when discussing the DPT reaction. If the GSDPT reaction proceeds in a concerted way, then such localization, delocalization, or both of the vibrational motion should be very important. The IR spectrum obtained in the present study suggests that the NH stretch level is energetically close to the barrier of the GSDPT reaction. To examine whether the GSDPT
J. Phys. Chem. A, Vol. 114, No. 9, 2010 3205 reaction is induced by the IR excitation, a detection of the normal dimer as a reaction product is necessary. Acknowledgment. This work is partially supported by the Grant-in-Aid for Scientific Research in Priority Areas (grant 19056004) and by the Grant-in-Aid for Scientific Research (grant 21550016) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. Supporting Information Available: Vibrational frequencies of the deuterated species of the 7-azaindole tautomeric dimer and the long-time temporal change in the fluorescence spectra. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Taylor, C. A.; El-Bayoumi, M. A.; Kasha, M. Proc. Natl. Acad. Sci. U.S.A. 1969, 63, 253. (b) Ingham, K. C.; Abu-Elgheit, M.; El-Bayoumi, M. A. J. Am. Chem. Soc. 1971, 93, 5023. (c) Ingham, K. C.; El-Bayoumi, M. A. J. Am. Chem. Soc. 1974, 96, 1674. (d) El-Bayoumi, M. A.; Avouris, P.; Ware, W. R. J. Chem. Phys. 1975, 62, 2499. (2) (a) Fuke, K.; Yoshiuchi, H.; Kaya, K. J. Phys. Chem. 1984, 88, 5840. (b) Fuke, K.; Kaya, K. J. Phys. Chem. 1989, 93, 614. (3) Hetherrington, W. M., III; Micheels, R. H.; Eisenthal, K. B. Chem. Phys. Lett. 1979, 66, 230. (4) (a) Bulska, H.; Chodkowska, A. J. Am. Chem. Soc. 1980, 102, 3259. (b) Bulska, H.; Grabowski, A.; Pakuła, B.; Sepioł, J.; Waluk, J.; Wild, U. P. J. Lumin. 1984, 29, 65. (5) (a) Tokumura, K.; Watanabe, Y.; Itoh, M. Chem. Phys. Lett. 1984, 111, 379. (b) Tokumura, K.; Watanabe, Y.; Udagawa, M.; Itoh, M. J. Am. Chem. Soc. 1987, 109, 1346. (6) Share, P.; Pereira, M.; Sarisky, M.; Repinec, S.; Hochstrasser, R. M. J. Lumin. 1991, 48/49, 204. (7) (a) Takeuchi, S.; Tahara, T. Chem. Phys. Lett. 1997, 277, 340. (b) Takeuchi, S.; Tahara, T. J. Phys. Chem. A 1998, 102, 7740. (c) Takeuchi, S.; Tahara, T. Chem. Phys. Lett. 2001, 347, 108. (8) Suzuki, T.; Okuyama, U.; Ichimura, T. J. Phys. Chem. A 1997, 101, 7047. (9) (a) Chachisvilis, M.; Fiebig, T.; Douhal, A.; Zewail, A. H. J. Phys. Chem. A 1998, 102, 669. (b) Fiebig, T.; Chachisvilis, M.; Manger, M.; Zewail, A. H.; Douhal, A.; Garcia-Ochoa, I.; De La Hoz Ayuso, A. J. Phys. Chem. A 1999, 103, 7419. (c) Kwon, O.-H.; Zewail, A. H. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 8703. (10) Catala´n, J.; Perez, P.; del Valle, J. C.; de Paz, J. L. G.; Kasha, M. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 419. (11) Douhal, A.; Kim, S. K.; Zewail, A. H. Nature 1995, 378, 260. (12) (a) Nakajima, A.; Ono, F.; Kihara, Y.; Ogawa, A.; Matsubara, K.; Ishikawa, K.; Baba, M.; Kaya, K. Laser Chem. 1995, 15, 167. (b) Nakajima, A.; Hirano, M.; Hasumi, R.; Kaya, K.; Watanabe, H.; Carter, C. C.; Williamson, J. M.; Miller, T. A. J. Phys. Chem. A 1997, 101, 392. (13) Lopez-Martens, R.; Long, P.; Sogaldi, D.; Soep, B.; Syage, J.; Millie, P. Chem. Phys. Lett. 1997, 273, 219. (14) (a) Folmer, D. E.; Poth, L.; Wisniewski, E. S.; Castleman, A. W., Jr. Chem. Phys. Lett. 1998, 287, 1. (b) Folmer, D. E.; Wisniewski, E. S.; Castleman, A. W., Jr. Chem. Phys. Lett. 2000, 318, 637. (15) (a) Sakota, K.; Hara, A.; Sekiya, H. Phys. Chem. Chem. Phys. 2004, 6, 32. (b) Sakota, K.; Sekiya, H. J. Phys. Chem. A 2005, 109, 2718. (c) Sakota, K.; Sekiya, H. J. Phys. Chem. A 2005, 109, 2722. (d) Sakota, K.; Okabe, C.; Nishi, N.; Sekiya, H. J. Phys. Chem. A 2005, 109, 5245. (16) (a) Sekiya, H.; Sakota, K. Bull. Chem. Soc. Jpn. 2006, 79, 385. (b) Sekiya, H.; Sakota, K. J. Photochem. Photobiol., C 2008, 9, 81. (17) (a) Douhal, A.; Guallar, V.; Moreno, M.; Lluch, J. M. Chem. Phys. Lett. 1996, 256, 370. (b) Douhal, A.; Moreno, M.; Lluch, J. M. Chem. Phys. Lett. 2000, 324, 75. (c) Douhal, A.; Moreno, M.; Lluch, J. M. Chem. Phys. Lett. 2000, 324, 81. (d) Moreno, M.; Douhal, A.; Lluch, J. M.; Castano, O.; Frutos, L. M. J. Phys. Chem. A 2001, 105, 3887. (18) (a) Catala´n, J.; Del. Valle, J. C.; Kasha, M. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 8338. (b) Catala´n, J.; Del Valle, J. C.; Kasha, M. Chem. Phys. Lett. 2000, 318, 629. (c) Del Valle, J. C.; Kasha, M.; Catala´n, J. Int. J. Quantum Chem. 2000, 77, 118. (d) Catala´n, J.; De Pazb, J. L. G. J. Chem. Phys. 2005, 123, 114302. (19) (a) Serrano-Andres, L.; Merchan, M.; Carlos Borin, A.; Stalring, J. Int. J. Quantum Chem. 2001, 84, 181. (b) Serrano-Andres, L.; Merchan, M. Chem. Phys. Lett. 2006, 418, 569. (20) Guallar, V.; Batista, V.; Miller, W. H. J. Chem. Phys. 1999, 110, 9922. (21) Baer, T.; Hase, W. L. Unimolecular Reaction Dynamics: Theory and Experiments; Oxford University Press: New York, 1996.
3206
J. Phys. Chem. A, Vol. 114, No. 9, 2010
(22) Stannard, P. R.; Gelbart, W. M. J. Phys. Chem. 1981, 85, 3592. (23) (a) Yokoyama, H.; Watanabe, H.; Omi, T.; Ishiuchi, S.; Fujii, M. J. Phys. Chem. A 2001, 105, 9366. (b) Yokoyama, H.; Watanabe, H.; Omi, T.; Ishiuchi, S.; Fujii, M. J. Phys. Chem. A 2002, 106, 854. (24) Sakai, M.; Ishiuchi, S.; Fujii, M. Eur. Phys. J. D 2002, 20, 399. (25) Sakai, M.; Fujii, M. Chem. Phys. Lett. 2004, 396, 298. (26) (a) Dwyer, J. R.; Dreyer, J.; Nibbering, E. T. J.; Elsaesser, T. Chem. Phys. Lett. 2006, 432, 146. (b) Elsaesser, T.; Huse, N.; Dreyer, J.; Dwyer, J. R.; Heyne, K.; Nibbering, E. T. J. Chem. Phys. 2007, 341, 175. (27) Dreyer, J. J. Chem. Phys. 2007, 127, 054309. (28) Lipert, R. J.; Colson, S. D. J. Phys. Chem. 1989, 93, 3894. (29) Ebata, T. Population Labeling Spectroscopy In Nonlinear Spectroscopy for Molecular Structure Determination; Field. R. W., Hirota, E., Maier, J. P., Tsuchiya, S., Eds.; Blackwell Science: Cambridge, U.K., 1998; pp 149-165. (30) Tanabe, S.; Ebata, T.; Fujii, M.; Mikami, N. Chem. Phys. Lett. 1993, 326, 347. (31) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.,; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishikawa et al. Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. GAUSSIAN 03, revision E.01; Gaussian, Inc.: Wallingford, CT, 2004. (32) Calculated vibrational frequencies of all deuterated dimers are listed in Table S1 in the Supporting Information of this article. (33) Stein, S. E.; Rabinovitch, B. S. J. Chem. Phys. 1973, 58, 2438. (34) In the case of the NH-ND dimer, the in-plane ND bend mode is coupled to the C-C bend modes, and the linear combinations among them are obtained as normal modes. Therefore, the exact frequency of the ND bend mode cannot be obtained.
JP909337W
